of H-bond acceptor and donor segments, dual amide groups, UPy moieties, and
DAT-DAT interactions create high-strength H-bonded hydrogels with self-healing
or self-recovery functions. Such hydrogels with 40–80 wt% water exhibit a Young’s
modulus up to 84 MPa and sustain 1–5 MPa tensile stresses at 800–1,400%
elongations. H-bonded hydrogels prepared by polymerization of NAGA monomer
with dual amide groups in aqueous solutions exhibit the highest stretchability, while
those formed by combination of hydrophobic and H-bonding interactions via UPy
and C18A units, respectively, have the highest tensile strength. DMAA and MAAc
are attractive H-bond acceptor and donor monomers, respectively, for the preparation of H-bonded hydrogels. As compared to AAm, DMAA has an enhanced
H-bond acceptor capability through their carbonyl groups via σ-donation effect of
the methyl groups contributing to the H-bonding cooperativity in hydrogels when it
is copolymerized with H-donor monomers. Moreover, the use of MAAc instead of
AAc as a H-bond donor monomer significantly improves the mechanical performance of H-bonded hydrogels due to the hydrophobic interactions of the α-methyl
groups of MAAc units. The primary chain length of H-bonded hydrogels is also
effective in determining their mechanical strength due to the proximity effect.
Increasing the chain length of the primary chains facilitates formation of H-bonds
in the vicinity of preexisting H-bonds contributing H-bond cooperativity leading to
high-strength hydrogels. Recently developed H-bonded hydrogels capable of
absorbing a large amount of water (~1,700 g g
À1 ) and those containing ds-DNA in
a clay environment are attractive self-healing soft materials for various applications
such as superabsorbent polymers and in gene delivery systems, respectively.
Hydrophobic interactions between hydrophobically modified polymers in an
aqueous environment lead to the formation of hydrogels containing crystalline
domains and/or hydrophobic associations acting as strong and weak physical
cross-links, respectively. Such hydrogels have recently been prepared using bulk,
solution, and micellar copolymerization of a variety of hydrophilic and hydrophobic
monomers via free-radical mechanism. The feed molar ratio of the monomers, alkyl
side chain length of the hydrophobes, type of the hydrophilic monomer, water
content, and the presence of surfactant micelles are the main experimental parameters providing precise control of the mechanical, viscoelastic, and self-healing
properties of the hydrogels. When the hydrophobe content is limited to 2 mol%,
SDS-containing self-healing hydrogels obtained from DMAA and C17.3M exhibit
the highest stretchability (4,200 Æ 400%) due to the associative behavior of DMAA
segments. They also exhibit a complete self-healing without any external stimuli at
24
C within 20 min. Replacing nonionic DMAA with the anionic AAc monomer
and SDS with the cationic CTAB surfactant produces physical hydrogels of high
tensile strength (1.7 MPa) due to the dual hydrophobic and ionic interactions.
Moreover, a significant mechanical property improvement in the hydrogels could
be achieved when the hydrophobe content is increased above 10 mol% providing
formation of crystalline domains in addition to the hydrophobic associations.
AAc/C18A comonomer produces semicrystalline hydrogels with the highest melting
temperature (48–56
C), degree of crystallinity (10–33%), Young’s modulus (up to
308 Æ 16 MPa), and tensile strength (up to 5.1 Æ 0.1 MPa). Damaged hydrogels with
54
O. Okay
DAT-DAT interactions create high-strength H-bonded hydrogels with self-healing
or self-recovery functions. Such hydrogels with 40–80 wt% water exhibit a Young’s
modulus up to 84 MPa and sustain 1–5 MPa tensile stresses at 800–1,400%
elongations. H-bonded hydrogels prepared by polymerization of NAGA monomer
with dual amide groups in aqueous solutions exhibit the highest stretchability, while
those formed by combination of hydrophobic and H-bonding interactions via UPy
and C18A units, respectively, have the highest tensile strength. DMAA and MAAc
are attractive H-bond acceptor and donor monomers, respectively, for the preparation of H-bonded hydrogels. As compared to AAm, DMAA has an enhanced
H-bond acceptor capability through their carbonyl groups via σ-donation effect of
the methyl groups contributing to the H-bonding cooperativity in hydrogels when it
is copolymerized with H-donor monomers. Moreover, the use of MAAc instead of
AAc as a H-bond donor monomer significantly improves the mechanical performance of H-bonded hydrogels due to the hydrophobic interactions of the α-methyl
groups of MAAc units. The primary chain length of H-bonded hydrogels is also
effective in determining their mechanical strength due to the proximity effect.
Increasing the chain length of the primary chains facilitates formation of H-bonds
in the vicinity of preexisting H-bonds contributing H-bond cooperativity leading to
high-strength hydrogels. Recently developed H-bonded hydrogels capable of
absorbing a large amount of water (~1,700 g g
À1 ) and those containing ds-DNA in
a clay environment are attractive self-healing soft materials for various applications
such as superabsorbent polymers and in gene delivery systems, respectively.
Hydrophobic interactions between hydrophobically modified polymers in an
aqueous environment lead to the formation of hydrogels containing crystalline
domains and/or hydrophobic associations acting as strong and weak physical
cross-links, respectively. Such hydrogels have recently been prepared using bulk,
solution, and micellar copolymerization of a variety of hydrophilic and hydrophobic
monomers via free-radical mechanism. The feed molar ratio of the monomers, alkyl
side chain length of the hydrophobes, type of the hydrophilic monomer, water
content, and the presence of surfactant micelles are the main experimental parameters providing precise control of the mechanical, viscoelastic, and self-healing
properties of the hydrogels. When the hydrophobe content is limited to 2 mol%,
SDS-containing self-healing hydrogels obtained from DMAA and C17.3M exhibit
the highest stretchability (4,200 Æ 400%) due to the associative behavior of DMAA
segments. They also exhibit a complete self-healing without any external stimuli at
24
C within 20 min. Replacing nonionic DMAA with the anionic AAc monomer
and SDS with the cationic CTAB surfactant produces physical hydrogels of high
tensile strength (1.7 MPa) due to the dual hydrophobic and ionic interactions.
Moreover, a significant mechanical property improvement in the hydrogels could
be achieved when the hydrophobe content is increased above 10 mol% providing
formation of crystalline domains in addition to the hydrophobic associations.
AAc/C18A comonomer produces semicrystalline hydrogels with the highest melting
temperature (48–56
C), degree of crystallinity (10–33%), Young’s modulus (up to
308 Æ 16 MPa), and tensile strength (up to 5.1 Æ 0.1 MPa). Damaged hydrogels with
54
O. Okay